Another interesting thing I noticed about this (our neighbor's house is being assembled in this way, and a couple more in the neighborhood) is that they'll hold off on window openings until after the roof is put up. So these wall sections are framed for windows inside, then sheathed as if there is no window there. After the roof goes up and shingles go on, they'll cut out the opening and throw the windows in. It's pretty genius as far as waterproofing a construction site goes.
The most common I've seen are wall sections which are studded and sheathed in a warehouse, sometimes with exterior covering as well (Hardie board most commonly), then delivered via crane truck. Trusses are also common.
Depends on the builder, really. Also the lumber supplier. There are many around me, they seem to offer this service as an advantage.
Builders who do this pre-assembly have a significantly quicker completion time, especially considering the weather, and their ability to get the roof and windows/doors in before the rains come.